Solid golf balls are classified into a two-piece golf ball, which is obtained by directly covering a core with a cover material, and a multi-layer golf ball, which is obtained by interposing one or more intermediate layers between a core and a cover. In the solid golf ball, an elastic portion including the core are formed from an elastic material obtained by vulcanization molding a rubber composition. The rubber composition for forming the elastic portion is generally composed of a base rubber (e.g. polybutadiene rubber, etc.), a monomer having an unsaturated bond (e.g. metal salt of .alpha.,.beta.-unsaturated carboxylic acid, etc.) as a co-crosslinking agent for improving a rebound coefficient and impact resistance of the golf ball and a peroxide as a co-crosslinking initiator. It is also known that the core imparts suitable hardness and durability and a solid golf ball using the core exhibits excellent durability and good flight performance, because a three-dimensionally crosslinked polymer is formed by the above mentioned rubber composition.
The solid golf ball attains long flight distance because of large ball velocity at the time of hitting, and shows good flight performance and good durability. However, the solid golf ball has problems in shot feel and controllability at approach shot, because shot feel at the time of hitting is hard and spin is not easily applied on the golf ball and the controllability of the ball at approach shot is poor, because a ball velocity at the time of hitting is large and an area in contact with a golf club is small.
In order to improve shot feel at the time of hitting, it is proposed that the golf ball is made softened by reducing the hardness of the core. In the solid golf ball, the softening of the core improves the shot feel at the time of hitting of the golf ball, but the rebound characteristics or initial velocity of the golf ball at the time of hitting are lowered, and sufficient flight distance is not obtained.
In addition, for improving spin performance, the cover material must be improved. This is because that the ionomer resin, which is generally used as a cover material of the solid golf ball, has high rigidity and hardness to satisfy the flight performance and durability of the golf ball, but spin performance is not satisfied.
In order to solve the problems, a resin having low rigidity and low hardness has been suggested as a cover resin for the solid golf ball. When the rigidity and hardness of the cover are lowered, the spin performance is effectively improved, but the rebound performance of the cover itself is deteriorated, instead. Therefore, satisfactory flight distance as the golf ball is not obtained.
Japanese Patent Kokai Publication No. 109970/1992 suggests that an organosulfur compound is added into the above rubber composition for core. However, the addition of the organosulfur compound makes the core softened and increases the compression value of the core. In order to adjust the compression value within the range necessary for the solid golf balls, an amount of the polymerization initiator or an amount of zinc acrylate as the co-crosslinking agent has to be increased, but this has a drawback increasing the producing cost.